Process for stabilizing the whiteness of lignin-containing fibrous materials, stabilizer composition and fibrous materials stabilized with this

ABSTRACT

Lignin-containing fibrous materials, in particular paper and card, are treated with whiteness stabilizers for the purpose of reducing light-induced yellowing. The whiteness stabilizer are (i) thiols with at least one additional hydrophilic group from the series consisting of OH, NH 2 , NHR 1 m NR 1 R 2 , COOH, SO 3 H and PO 3 H 2  and an amino acid/peptide structure (ii) thiols with a CC double bond (iii) or heteroaromatic thiols and (iv), heteroaromatic disulfides or polysulfides. HALS light stabilizers and/or UV stabilizers are advantageously additionally employed. The stability is increased further by a TiO 2 -containing coat on the fibrous material. Stabilzer compositions and fibrous material stabilized according to the invention, containing a whiteness stabilizer of group (i) to (iv) are also disclosed.

INTRODUCTION AND BACKGROUND

[0001] The present invention relates to a process for stabilizing the whiteness of lignin-containing fibrous materials, in particular paper and cardboard of mechanically produced fibrous materials, in particular wood pulp, against light-induced yellowing. The process comprises the use of a sulfur-containing whiteness stabilizer. The invention furthermore relates to whiteness-stabilized lignin-containing fibrous materials obtainable according to the invention and to a stabilizer composition.

[0002] The low stability to whiteness of lignin-containing high-yield fibrous materials such as are obtainable using mechanically produced fibrous substances, that is to say in particular fibrous substances produced thermomechanically (TMP), chemo-thermomechanically (CTMP) or by stone-grinding of wood (SGW), is a serious obstacle to employing high-yield fibrous substances obtained in this way, in particular wood pulps, for the production of high-finish paper and card products. Bleached lignin-containing wood pulps and celluloses in particular are highly susceptible to light-induced yellowing.

[0003] The causes and prevention of light-induced yellowing of celluloses are reported by J. S. Gratzl in “Das Papier” 39/10 A (1985), pages V16 to V23. For increasing the whiteness and improving the light stability of celluloses, in particular lignin-rich high-yield celluloses, reductive measure are suitable, such as treatment of the cellulose with sodium boranate or the use of a combination of formaldehyde with a dithionite salt or addition of tin(II) salts. The tendency towards yellowing can furthermore be reduced effectively by using a UV filter, for example a 2-hydroxybenzophenone or 2-hydroxybenzotriazole.

[0004] In the context of mechanistic studies by Dian Yan Lee et al. (Holzforschung 45 (1991) suppl. pages 15 to 20), it was found that 1-thioglycerol is active to a certain extent in both the bleaching and the stabilization of the whiteness of mechanically produced fibrous substances. A disadvantage of thioglycerol is its low molecular weight and therefore also its unpleasant smell, which impedes use in practice.

[0005] C. Heitner (“Pulp Bleaching-Principle and Practice”, C. W. Dence, D. W. Reeve, eds., TAPPI PRESS, Atlanta, 1996, pages 183 to 211) gives an overview of the prior art on yellowing of lignin-containing fibrous substances. The use of phosphites, UV absorbers and agents which trap free radicals, such as ascorbic acid, thiols and thioethers, is reported. It is stated in WO 99/05108 that the substances mentioned in this document are still not an adequate and practically applicable solution for avoiding light-induced yellowing.

[0006] According to another study of light- and heat-induced yellowing of fibrous substances (M. Beyer, Ch. Bäurich and K. Fischer in “Das Papier”, volume 10A (1995), pages V8 to V14) yellowing can be counteracted by targeted application of a combination of ascorbic acid with a compound containing a thiol group. A further improvement is achieved by applying a pigmented coat to a paper comprising the combination mentioned. This document does not reveal what structure the thiol must have to effect effective resistance to yellowing.

[0007] According to U.S. Pat. No. 5,942,567, yellowing of casting resins can be reduced by using a combination of a phenolic antioxidant and a non-aromatic thiol compound. The thiols are alkylmercaptans, such as, in particular, hexylmercaptan, dithioglycol and 3-mercaptopropyltriethoxysilane, all of which are an odour nuisance. No reference to the use of such a combination as a whiteness stabilizer for lignin-containing fibrous materials, such as paper, is to be found in this document.

[0008] The teaching of U.S. Pat. No. 6,059,927 is a process for stabilizing the whiteness of mechanically produced bleached wood pulps. For this, a combination of an aqueous formaldehyde solution with a carbonate is employed in the wood pulp substance; a further increase in the resistance to yellowing is effected by using a polyethylene glycol bisthiol, polyethylene glycol bisthiolactate and polyethylene glycol bisthioglycollate being mentioned exclusively as the thiol compound. The amount of these thiols employed is in the range from 2 to 6 wt. %, based on the wood pulp. The high amount of the said bisthiols employed is a disadvantage of this process. No indication that these bisthiols can be employed in the absence of formaldehyde and carbonate for stabilizing whiteness can be found in this document.

[0009] Studies by B. J. W. Cole at al. (Journal of Wood Chemistry and Technology, 20 (1), 1-17 (2000)) on the bleaching and photostabilization of high-yield fibrous substances show that ethylene glycol bismercaptoacetate (=ethylene glycol dithioglycollate) has a bleaching and stabilizing action, while pentaerythrytol tetrathioglycollate has no bleaching action and leads to only a slight decolorization in the system investigated.

[0010] Effective stabilization of the whiteness of TMP and CTMP fibrous substances or papers produced therefrom is effected according to WO 99/05108 by the use of derivatives of 1-oxyl-2,2,6,6-tetramethylpiperidin-4-ol and selected hydroxylamine salts. The action is increased by additional use of co-additives, such as UV absorbers, antioxidants, nitrones, fluorescent brighteners and sulfur-containing stabilizers. The sulfur-containing stabilizers, which are employed only in combination with an N-oxide or hydroxylamine light stabilizer, are compounds with a thiol or thioether group. The following thiols, which are predominantly compounds presenting an odour nuisance, are mentioned: thioglycerol, dithiothreitol, 2,2′-oxydiethanethiol, 2-(2′-methoxyethoxy)-ethanethiol, thioglycollic acid, 2- and 3-mercaptopropionic acid and Na salts thereof, diesters of the mercaptocarboxylic acid mentioned with ethylene glycol and polyethylene glycol and trimethylolpropane tri(3-mercaptopropionate).

[0011] It is therefore an object of the present invention is to improve stabilizing the whiteness of lignin-containing fibrous materials against light-induced yellowing and to enable producing paper and card with improved resistance to yellowing.

[0012] According to another object the whiteness stabilizers to be used according to the invention should have the lowest possible smell and show a high activity by themselves, and in particular in combination with other known whiteness stabilizers or co-additives at the lowest possible total concentration of additives.

[0013] A further object of the invention is aimed at providing a composition comprising one or more whiteness stabilizers, the use of which in the pulp of the fibrous substance or in a composition for impregnating or coating, such as a size or a so-called coat, the leads to a high stabilization of the whiteness of fibrous material.

SUMMARY OF THE INVENTION

[0014] The above and other objects of the present invention can be achieved by or process which uses a whiteness stabilizer selected from the group consisting of

[0015] (i) thiols which have at least one aliphatically or cycloaliphatically bonded thiol group and additionally at least one further hydrophilic group selected from the group consisting of —OH, —NH₂,—NHR¹, —NR¹R², wherein R¹ and R² represent C₁- to C₄-alkyl or C₂- to C₄-hydroxyalkyl, such as hydroxyethyl and dihydroxypropyl, C₂- to C₄-alkanoyl- or -hydroxyalkanoyl, or taken together represent cycloalkyl, or —COOH, —SO₃H and —PO₃H₂ and salts of the acid and base function,

[0016] (ii) thiols which contain at least one aliphatically or cycloaliphatically bonded thiol group and additionally at least one CC double bond,

[0017] (iii) thiols which contain at least one thiol group bonded to a heteroaromatic, and

[0018] (iv) di- and polysulfides with at least one heteroaromatic bonded to the di-/polysulfide group,

[0019] in the production of the fibrous material, that is to say in particular paper and card, with the proviso that thioglycerol, dithiothreitol mercaptoacetic acid, 2- and 3-mercaptopropionic acid, and salts therefor being excluded as the whiteness stabilizer.

[0020] The term “lignin-containing fibrous materials” is understood as meaning all materials which comprise a lignin-containing fibrous substance as the essential constituent. The fibrous substances are, in particular, lignin-containing wood pulps, but lignin-containing celluloses are also suitable for use in the process according to the invention. Stabilization of the whiteness and therefore prevention of yellowing is of industrial importance in particular in so-called high-yield wood pulps which are obtained thermomechanically (TMP) or chemo-thermomechanically (CTMP) or by stone-grinding (SGW). The raw materials mentioned are inexpensive raw materials which, if the whiteness stabilization is adequate, can be employed both in papers of medium quality and in high-quality papers and cards. The process according to the invention is also particularly suitable for stabilizing the whiteness of bleached wood pulps, which tend to yellow in particular without stabilization.

[0021] According to the invention, the resistance to yellowing of lignin-containing fibrous substances and the materials produced therefrom can be increased and the whiteness thus stabilized if a thiol compound according to the invention is employed in an active amount during the production of the fibrous material. The amount employed depends on the desired degree of stabilization and on whether and to what extent further whiteness stabilizers and/or co-additives are employed in the production of the fibrous material. The amount of thio compound according to the invention employed is conventionally in the range from 0.01 to 5 wt. %, preferably 0.1 to 3 wt. % and particularly preferably 0.5 to 2 wt. %, in each case based on the dry fibrous substance.

DETAILED DESCRIPTION OF INVENTION

[0022] The whiteness stabilizers according to the invention from group (i) are thiols which have at least one aliphatically or cycloaliphatically bonded thiol group and additionally at least one hydrophilic group according to the claims. Such thiols can have various structures, such as, for example:

[0023] Mercaptoalcohols of the general formula (HS)_(m)X(OH)_(n), wherein X represents an optionally alkyl-substituted cycloaliphatic, a linear or branched hydrocarbon radical having 4 to 20 C atoms, in particular 5 to 8 C atoms, and m and n represent an integer from 1 to 5. Examples are di-, tri- and tetrathiopentitols and -hexitols.

[0024] Mercaptoalcohols of the general formula (HS)_(m)Y(OH)_(n), wherein Y represents a linear or branched hydrocarbon radical which is interrupted once or several times by a —O—, —S—, —NH— or N-alkyl bridge and has 4 to 20 C atoms in total and m and n have the abovementioned meaning. Examples are reaction products of polyhydric alcohols with ethylene sulfide or a mercaptoalkyl halide, at least one hydroxyl group remaining on the hydrocarbon radical.

[0025] Mercaptoalcohols of the general formula (HS(CH₂)_(p)—COO)_(m)Z(OH)_(n), wherein Z has the meaning of X or Y or represents a polymeric hydrocarbon chain, m and n have the above mentioned meaning or in the case of a polymeric matrix can also go far beyond the values mentioned and p represents 1 or 2, wherein the mercapto group can be bonded internally or terminally. These are preferably an ester of a polyhydric alcohol, such as ethylene glycol, di-, tri- and polyethylene glycol, propane-1,2 and -1,3-diol, butane-1,4-diol, hexane-1,2,6-triol, trimethylolethane (TME), trimethylolpropane (TMP), erythritol, pentaerythritol (penta), di-TME, di-TMP, di-penta, sugar alcohols, sugar acids and lactones thereof with a mercaptocarboxylic acid from the series consisting of mercaptoacetic acid (=thioglycollic acid), 2- and 3-mercaptopropionic acid, mono- and dimercaptosuccinic acid. The polyhydric alcohol can also be partly esterified with a mercaptocarboxylic acid other than those mentioned above. The polymeric alcohols esterified with a mercaptocarboxylic acid are, for example, an oligo- or polysaccharide, such as starch, or polyvenyl alcohol.

[0026] Particularly suitable mercaptoalcohols of the abovementioned type are the di- or triester of pentaerythritol with mercaptoacetic acid or 1 or 2-mercaptopropionic acid, in particular pentaerythritol trithiolglycollate, and the di-, tri- or tetraester of a pentitol or hexitol, in particular sorbitol tetrathioglycollate. It has been found that such compounds have a considerably better action as a whiteness stabilizer than the compounds which are already known and have a similar structure but no longer contain a free hydroxyl group.

[0027] Mercaptoamines which contain, as hydrophilic group(s), at least one NH₂, NHR¹ or NR¹R² group, wherein R¹ and R² can be identical or different and can represent a lower alkyl group, such as methyl, ethyl, propyl, hydroxyethyl, hydroxypropyl or dihydroxypropyl or can be bonded together, on a linear, cycloaliphatic or branched carbon skeleton, which can be interrupted once or several times by —O—, —S—, —NH— or N-alkyl, also have a stabilizing action. Examples are N-mercaptoethyl-diethylenetriamine and N-mercaptoethyl-diethanolamine, and salts of these amines.

[0028] Thiols with an acid function as the hydrophilic group are, for example, mercaptosuccinic acid, dimercaptosuccinic acid, mercaptopropanesulfonic acid, mercaptoethyaminomethanephosphonic acid. Salts, in particular alkali metal salts, of the acids mentioned are also active.

[0029] A particularly preferred group of thiols of group (i) are mercaptoamino acids, such as, in particular, cysteine, N-acylated mercaptoamino acids, such as acetylcysteine, and di- and oligopeptides with at least one amino acid containing mercapto groups. A particularly good whiteness stabilizer, which moreover is completely odour-free and is highly active especially with co-stabilizers, is the tripeptide glutathione (γ-glutamylcysteineglycine).

[0030] It has been found that the yellowing-inhibiting action of mercapto compounds is increased by additionally containing at least one olefinic CC double bond (group (ii)). These are preferably compounds which have an allyl structure. Examples are esters of allyl alcohol or cinnamyl alcohol with a mercaptocarboxylic acid having 2 to 4 C atoms, such as thioglycollic acid, thiolactic acid and mercaptosuccinic acid.

[0031] Surprisingly, thiols which contain at least one mercapto group on a heteroaromatic have proven to be an active yellowing inhibitor (group(iii)). The heteroaromatics are 5- and 6-membered heteroaromatics with optionally another fused-on aromatic or heteroaromatic ring system. They contain one, two or three heteroatoms from the series consisting of O, S and N, rings with two or three heteroatoms being preferred, such as those selected from the group consisting of s-triazine, thiadiazole, thiazole, diazole. Such heteroaromatics optionally contain, in addition to at least one mercapto group, one or two amino or alkylated amino groups, alkylmercapto or alkoxy groups. They are particularly preferably monomercapto-diamino-s-triazines and dimercapto-amino-s-triazine, it being possible for amino groups to be substituted by one or two C₁- to C₃-alkyl groups or allyl groups or to be part of a five- or six-membered ring. Examples are: 2-mercapto-4-allylamino-6-diethylamino-s-triazine, 2,4-dimercapto-6-allylamino-s-triazine and 2-mercapto-4-ethylamino-6-morpholino-s-triazine. Finally, trimercapto-s-triazine, in particular in the form of a mono-, di- and tri-alkali metal salt, also has a whiteness-stabilizing action. The mercaptotriazine compounds can be obtained in a manner known per se by stepwise reaction of cyanuric chloride with one or two amines and sodium hydrogen sulfide.

[0032] Whiteness stabilizers of group (iv) contain one or more di- or polysulfide groupings on a heteroaromatic of the type already mentioned above or between two heteroaromatics. Polysulfide groups are also understood as meaning tri- and tetrasulfides. The second function of a polysulfide can be satisfied by hydrogen or an alkali metal, or by oligomerization.

[0033] Examples are 2,4-di-polysulfido-6-diethylamino-s-triazine, bis-2-methylamino-1,3,4-thiadiazin-5-yl disulfide and bis-[2,4-di(dialkylamino)-triazin-6-yl] tetrasulfide. The polysulfidotriazine is prepared e.g. by reaction of 2-diethylamino-4,6-dimercapto-2-triazine with S₂Cl₂ in petroleum ether; thiadiazinyl disulfide is obtainable by an oxidation of methylamino-mercapto-1,3,4-thiadiazole with H₂O₂, and the tetrasulfide mentioned can be obtained by reaction of the corresponding bis(dialkylamino)-triazinylmercapto compound with S₂Cl₂.

[0034] The action of whiteness stabilizers to be used according to the invention which have one or more thiol groups or di-/polysulfide groups can be increased by the co-use of known whiteness stabilizers or co-stabilizers. These are substances selected from the group consisting of light stabilizers, in particular those selected from the group consisting of sterically hindered nitroxides and hydroxylamines and salts thereof (=HALS stabilizers), UV absorbers, antioxidants, fluorescent brighteners, agents which trap free radicals, metal deactivators, phosphites, aromatic sulfides and reducing agents other than thiols.

[0035] The HALS light stabilizers are compounds such as are mentioned by way of example in WO 99/05108 and U.S. Pat. Nos. 5,496,875 and 6,080,864. Suitable compounds are e.g. 1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine; 1-hydroxy-2,2,6,6-tetramethyl-4-hydroxypiperidinium chloride or a carboxylic acid salt according to this type, such as tetra(1-hydroxy-2,2,6,6-tetramethyl-4-hydroxypiperidinium) ethylenediaminetetraacetate; 1-oxyl-2,2,6,6-tetramethyl-4-alkyloxypiperidine; octadecyl-2,2,6,6-tetramethyl-4-piperidinyl carbonate. The amount of such stabilizers employed is expediently in the range from 0.01 to 2 wt. %. Combinations of thiols according to the invention, in particular those from group (i), such as mercaptocarboxylic acid esters of polyhydric alcohols, amino acids or oligopeptides containing thiol groups, with HALS light stabilizers lead to a synergistic effect in respect of the whiteness stabilization, so that the total amount of stabilizers can be reduced, or the activity can be increased noticeably by means of a small addition of a HALS stabilizer, such as 0.02 to 0.2 wt. %.

[0036] The UV absorbers are known substances selected from the group consisting of benzotriazoles and benzophenones, such as 3-tert-butyl-4-hydroxy-5-(2H-benzotriazol-2-yl)-hydrocinnamic acid and 2-(2-hydroxy-3,5-di-tert-butylphenyl)-2H-benzotriazole; 2,4-dihydroxybenzophenone and 2,2-dihydroxy-4,4-dimethoxy-5,5-disulfobenzophenone di-Na salt. A further class of UV absorbers which are suitable for stabilizing the whiteness of lignin-containing fibrous substances in the absence of whiteness stabilizers according to the invention comprises substances represented by the structural of the general formula:

[0037] wherein:

[0038] R¹: denotes H, (C₁-C₁₂)-alkanoyl, in particular acetyl, propionyl, n-butanoyl; (C₁-C₁₂)alkoxycarbonyl, in particular —COOCH₃, —COOC₂H₅,

[0039] R²: denotes (C₁-C₁₂)-alkyl, linear or branched, in particular methyl, ethyl, n-propyl, n-butyl, i-propyl

[0040] Ar—OH: denotes hydroxy-substituted aryl radical, wherein at least one hydroxyl group is bonded in the ortho- or para-position of the aromatic bonded to the olefinic double bond, in addition further substituents, such as alkyl, in particular methyl, ethyl, propyl, n-butyl, i-propyl, sec-butyl, tert-butyl, alkoxy, in particular methoxy and ethoxy, phenyl, p(o)-hydroxyphenyl, benzoyl, —COOH and SO₃H, can be bonded to the aromatic, and wherein Ar represents the benzene or naphthalene ring.

[0041] 3-Methoxy-4-hydroxybenzylidene-malonic acid esters and 3-(3-methoxy-4-hydroxybenzylidene-pentane-2,4-dione are particularly suitable.

[0042] The antioxidants are usually sterically hindered phenols, such as 2,6-di.-tert-butyl-4-methylphenol, hydroxylated thiodiphenyl ethers, triazines, such as 2,4-bis(octylmercapto)-6-(3,5-di-tert-butyl-4-hydroxyanilino-1,3,5-triazine.

[0043] Phosphites and phosphonites, in particular sterically hindered compounds, such as tris(2,4-di-tert-butylphenyl) phosphite, assist the whiteness stabilization, since they act as agents which trap peroxides and free radicals.

[0044] Various hydrazine derivatives, such as isophthaloyl dihydrazide and N,N-bis(salicyloyl)hydrazine, are active as metal deactivators. A further class comprises complexing agents from the series consisting of citrates, gluconates, keto acids, aminocarboxylic acids, such as EDTA, DTPA and phosphonates, such as hydroxyethane-diphosphonate (HEDP).

[0045] The fluorescent brighteners are, in particular, known substances selected from the group consisting of stilbene derivatives, such as 4,4′-(diphenyl)-stilbene and 4,4′-bis-(triazinylamino)-stilbene-2,2′-disulfonic acid.

[0046] Polymeric inhibitors, such as polyethylene glycol, polyvinylpyrrolidone and polytetrahydrofuran, are also suitable co-stabilizers.

[0047] One or more thiols or di-/polysulfides according to the invention in combination with a HALS light stabilizer or other agents which trap free radicals and a UV absorber selected from the group consisting of benzotriazoles or benzophenones and optionally additionally or alternatively an antioxidant, in particular a sterically hindered phenol, are particularly preferably employed, since a synergistic effect is fully effective here. Examples of particularly active combinations are: glutathione or sorbitol tetrathioglycollate, 2,2,6,6-tetramethyl-4-hydroxypiperidine N-oxide or a derivative thereof and 2-(2-hydroxyphenyl)-benzotriazole.

[0048] One or more whiteness stabilizers to be used according to the invention selected from the group consisting of thiols or di-/polysulfides or a combination of these compound(s) with one or more co-stabilizers selected from the group consisting of the abovementioned substance classes can be employed at various points in the production process of the fibrous material. It is possible to add the whiteness stabilizer or stabilizers and co-stabilizers individually or in the form of a composition to the mass of the fibrous substance, which is present e.g. in the form of a pulp. The addition is possible e.g. before, during or after a bleaching stage which is optionally carried out, or during the dewatering of the fibrous material.

[0049] In an alternative and usually preferred embodiment, the whiteness stabilizer or a composition comprising it is sprayed in the form of a solution or suspension on to the fibrous material, which has been at least partly dewatered. This spraying process is then followed by the sizing and/or coating stages well known in paper and card production.

[0050] According to another preferred alternative, the whiteness stabilizer to be used according to the invention or a composition comprising it is introduced into the medium used for sizing or coating the paper or card and then applied to the fibrous material.

[0051] It has been found that a considerable intensification of the yellowing-inhibiting effect of the whiteness stabilizers to be used according to the invention and composition comprising such can be achieved by applying a pigment-containing coat, in particular containing titanium dioxide, after application of the whiteness stabilizer, for example by impregnation or together with the size application. According to a particularly preferred embodiment, the whiteness stabilizer or stabilizers and optionally co-stabilizers are applied to the fibrous material together with titanium dioxide and optionally other constituents of a size or coating and the material is processed to the finished paper or card in a manner known per se.

[0052] The invention also provides a whiteness-stabilized lignin-containing fibrous material, in particular paper and card of mechanically produced wood pulp or high-yield cellulose, which can be bleached or unbleached, which comprises a whiteness stabilizer selected from the group consisting of the thiols, di or polysulfides of groups (i) to (iv) to be used according to the invention. The whiteness stabilizer here can be either in the pulp or in a size or coating or in a surface layer of the fibrous material produced by impregnation.

[0053] According to a particularly preferred embodiment of the whiteness-stabilized lignin-containing fibrous material according to the invention, this comprises a coating-which can be the size or a coating with a brush-on color which comprises titanium dioxide. A TiO₂ content in the range from 0.1 to 5 wt. %, based on the dried coating, leads to a significant increase in the resistance to yellowing.

[0054] As has been explained in detail in the description of the process according to the invention, in addition to one or more whiteness stabilizers according to the invention, the pulp of the fibrous material or a size or coating on this can also comprise other known whiteness stabilizers and/or co-stabilizers.

[0055] The invention also provides a composition which comprises at least one of the whiteness stabilizers to be used according to the invention from group (i) to (iv) in an aqueous, organic or aqueous-organic liquid or paste-like medium which can be used for spraying or coating. The composition preferably comprises one or more of the preferred stabilizers described above. According to a particularly preferred embodiment, the composition additionally comprises one or more other whiteness stabilizers/co-stabilizers selected from the group consisting of agents which trap free radicals, such as HALS light stabilizers, antioxidants, UV absorbers, reducing agents, fluorescent brighteners, metal deactivators, phosphites and aromatic sulfides. According to a particularly preferred embodiment, the composition comprises the combination of a whiteness stabilizer according to the invention, a HALS light stabilizer and/or a UV absorber and/or an antioxidant. Examples of such compositions have already been mentioned above. The composition expediently comprises at least one whiteness stabilizer according to the invention, a HALS light stabilizer and a UV absorber in a weight ratio of 1 to (0.02 to 0.5) to (0.2 to 2), in particular 1 to (0.05 to 0.2) to (0.5 to 2). A preferred composition is a brush-on color for coating a fibrous material. For the purpose of white coarse sand stabilization, such a brush-on color particularly preferably comprises, in addition to conventional base constituents, a combination of at least one white coarse sand stabilizers according to the invention at least one UV absorber and at least one agent which traps free radicals.

[0056] Particularly preferred compositions additionally comprise titanium dioxide, in addition to at least one whiteness stabilizer according to the invention. Compositions which are used for sizing purposes or a final coat additionally comprise conventional sizing components or binders and further inorganic pigments, additives and dispersing auxiliaries.

[0057] The present invention thus provides a large number of further highly active whiteness stabilizers of prevention of yellowing of lignin-containing fibrous materials, such as, in particular, paper and card. The whiteness stabilizers can be used by themselves, but preferably in combination with known whiteness stabilizers and/or co-stabilizers, synergistic effects being achieved thereby. A further increase in the resistance to yellowing is effected by the additional use of titanium dioxide in the size or in a coat. The subject matter according to the invention allows mechanically produced fibrous substances to be employed for the production of high-quality papers and cards, since the risk of light-induced yellowing is reduced decisively by the use of whiteness stabilizers according to the description herein.

[0058] The invention is illustrated with the aid of the following examples.

EXAMPLES

[0059] In each case a TCF-bleached aspen CTMP cellulose with a whiteness of 86% was used as the paper base

[0060] 1. Individual substances

[0061] The substances shown in table 1 were applied to the paper in the form of a solution. The amount applied was 1% by weight, calculated with respect to the cellulose. After drying, irradiation was carried out. Irradiation time was 10 h in a xenon test apparatus, level 2. The whiteness (W) (%) as a function of time was determined; the loss in whiteness results from this as the difference (=dW) between the whiteness before (=W₀) and after (W_(10h))the irradiation or as the relative drop in whiteness dW.100/W₀. TABLE 1 W₀ (%) before W₁₀ (%) W₀- Number Stabilizer irradiation after 10 h W₁₀ CE 1 None 86 55 31 Ex. 1 2-diallylamino-4-(3′- 88 67 21 Methoxypropyl) amino- 6-mercapto-1,3,5- triazine Ex. 2 2-N-piperidino-4- 87 65 22 Ethylamino-6- mercapto-1,3,5- trazine Ex. 3 2,2′-bis-(5- 81 61 20 methylamino-1,3,4- thiadiazolyl) disulfide CE 2 2,2,6,6-tetramethyl- 78 56 22 4-hydroxypiperidine N-oxide (is coloured!) Ex. 4 sorbitol 83 58 25 tetrathioglycollate Ex. 5 pentaerythritol 83 58 25 triatiolglycollate CE 3 pentaerythritol 86 54 32 tetra-thioglycollate

[0062] Examples 1 to 5 show the good action of thiols according to the invention. The action is the same or better than that of the HALS stabilizer of comparison example CE 2. Comparison of example 5 with comparison example CE 3 shows the unexpected influence of the additional hydroxyl group.

[0063] 2. Combinations

[0064] Combinations of a whiteness stabilizer (WS) according to the invention according to table 2a and b (1 wt. %), the HALS light stabilizer (agent which traps free radicals=FRT) 2,2,6,6-tetramethyl-4-hydroxypiperidine N-oxide (0.1 wt. %) and the UV absorber (UV) 2-(2-hydroxyphenyl)-benzotriazole (1 wt. %) were investigated. The use and testing were carried out analogously to those described under 1. The relative decrease in whiteness (%) after irradiation for 10 h and 20 h is shown in table 2b. TABLE 2a W₀ (%) before W₁₀ (%) W₀- Number Combination irradiation after 10 h W₁₀ CE 1 no WS, FRT and UV 86 55 31 CE 4 no WS; 87 68 19 with FRT and UV Ex. 6 sorbitol tetrakis- 87 75 12 thioglycollate and FRT and UV

[0065] TABLE 2b $\frac{\left( {W_{t} - W_{0}} \right) \cdot 100}{W_{0}}$

after 10 h after 20 h Number Combination (%) (%) CE 1 no WS, FRT and UV 33 42 CE 4 no WS, with FRT and 22 31 UV Ex. 6 sorbitol tetrakis- 14 22 thioglycollate with FRT and UV Ex. 7 cinnamyl meroapto- 19 26.5 glycollate with UV and FRT

[0066] 3. Combinations and TiO₂ in a Coat

[0067] Combinations comprising whiteness stabilizers (WS) according to the invention the UV (UV) absorber mentioned under 2. and an agent which traps free radicals (FRT) were investigated. The amount employed, where present, as before 1% WS, 0.1% FRT, 1% UV, in each cased based on the fibrous substance). After application of the particular stabilizer composition and drying, a TiO₂-containing coat was applied.

[0068] Composition of the brush-on color: 5 parts of inorganic pigment, of which 4 parts of calcium carbonate, 0.9 part of kaolin, 0.1 part of titanium dioxide, 3.5 parts of binder, 3.5 parts of water, dispersing auxiliary. Coat thickness 12 g/m².

[0069] The results follow from table 3a: Stabilizer-free comparison example CE 5 stabilizer-free paper was coated only with the stated brush-on color. Stabilizer-free paper. The paper according to example 8 contained, under the coat, an impregnation with glutathione (1 wt. %, based on the dyestuff), and that of example 9 additionally FRT 0.1 wt. % and UV (1 wt. %). The comparison shows the increase in action on changing from the coat-free, non-stabilized system (CE 1) to the system with a TiO₂-containing coat (CE 5), to the system with glutathione under a TiO₂-containing coat (Ex. 8) and finally a TiO₂-containing coat on a paper with an impregnation comprising glutathione, FRT and UV. TABLE 3 a W₁₀ (%) W₀- after W₁₀ Number Stabilizer; coat W₀ (%) 10 h (%) CE 1 no stabilizer, no 86 55 31 TiO₂ CE 5 no stabilizer, but 88 68 20 coat with TiO₂ Ex. 8 Glutathione + coat 85 72 13 with TiO₂ Ex. 9 Glutathione and 85 80 5 FRT and UV and coat with TiO₂

[0070] Examples Ex. 10 and Ex. 11 in table 3 illustrate the influence of TiO₂ in the coat. The coat composition in E 10 contained no TiO₂ by 1 part of kaolin, in example 11 0.7 part of kaolin and 0.3 part of TiO₂; the other constituents corresponded to the coat composition described above. TABLE 3 b W₁₀ (%) W₀- after W₁₀ Number Stabilizer; coat W₀ (%) 10 h (%) Ex. 10* Pentaerythritol 83.5 77.5 6 tris- thioglycollate and FRT and UV and coat without TiO₂ Ex. 11* pentaerythritol 86.5 82.5 4 tris- thioglycollate and FRT and UV and coat with TiO₂

[0071] Example 10 and 11 illustrate the influence of TiO₂ in the coat.

[0072] Further variations and modifications of the foregoing will be apparent to those skilled in the art and are intended to be encompassed by the claims appended hereto.

[0073] German priority application 101 12 101.6 is relied on and incorporated herein by reference. 

We claim:
 1. A process for stabilizing the whiteness of a lignin-containing fibrous material against light-induced yellowing, comprising contacting said fibrous material with a thio compound as a whiteness stabilizer in an active amount for increasing the resistance to yellowing during the production of the fibrous material, said thio compound being selected from the group consisting of (i) a thiol which has at least one aliphatically or cycloaliphatically bonded thiol group and additionally at least one further hydrophilic group selected from the group consisting of —OH, —NH₂, —NHR¹, —NR¹R², wherein R¹ and R² represent C₁- to C₄-alkyl or C₂ to C₄-hydroxyalkyl, C₂- to C₄-alkanoyl- or -hydroxyalkanoyl, or together represent cycloalkyl, or —COOH, —SO₃H and —PO₃H₂ and salts of the acid and base function, (ii) a thiol which contains at least one aliphatically or cycloaliphatically bonded thiol group and additionally at least one CC double bond, (iii) a thiol which contains at least one thiol group bonded to a heteroaromatic, and (iv) a di- and polysulfide with at least one heteroaromatic bonded to the di-/polysulfide group, with the proviso that thioglycerol, dithiothreitol mercaptoacetic acid, 2- and 3-mercaptopropionic acid, and salts thereof are excluded as the whiteness stabilizer.
 2. The process according to claim 1, wherein the fibrous material is selected from the group consisting of substantially mechanically, thermomechanically or chemo-thermomechanically produced lignin-containing unbleached and bleached fibrous substances and mixtures thereof.
 3. The process according to claim 2 wherein the fibrous material is paper or card from wood pulp.
 4. The process according to claim 1 further comprising adding said whiteness stabilizer to pulp of the fibrous material or to a size or brush-on color composition to be applied to a fibrous material, or a solution or suspension of a composition comprising the whiteness stabilizer is sprayed on to non-coated fibrous material.
 5. The process according to claim 2 further comprising adding said whiteness stabilizer to pulp of the fibrous material or to a size or brush-on color composition to be applied to a fibrous material, or a solution or suspension of a composition comprising the whiteness stabilizer is sprayed on to non-coated fibrous material.
 6. Process according to claim 1 further comprising adding to the fibrous material in addition to said thiol compound whiteness stabilizer or co-stabilizer selected from the group consisting of UV absorber, antioxidant, a metal deactivator, agent which trap free radicals, fluorescent brightener, phosphite, reducing agent and aromatic sulfide.
 7. Process according to claim 2 further comprising adding to the fibrous material in addition to said thiol compound whiteness stabilizer or co-stabilizer selected from the group consisting of UV absorber, antioxidant, a metal deactivator, agent which trap free radicals, fluorescent brightener, phosphite, reducing agent and aromatic sulfide.
 8. Process according to claim 4 further comprising adding to the fibrous material in addition to said thiol compound whiteness stabilizer or co-stabilizer selected from the group consisting of UV absorber, antioxidant, a metal deactivator, agent which trap free radicals, fluorescent brightener, phosphite, reducing agent and aromatic sulfide.
 9. The process according to claim 1 wherein a fibrous material treated with a whiteness stabilizer in the pulp, in a surface coating or by means of impregnation is coated with a brush-on color comprising titanium dioxide, or a non-treated fibrous material is coated with a brush-on color comprising a whiteness stabilizer and titanium dioxide.
 10. The process according to claim 2 wherein a fibrous material treated with a whiteness stabilizer in the pulp, in a surface coating or by means of impregnation is coated with a brush-on color comprising titanium dioxide, or a non-treated fibrous material is coated with a brush-on color comprising a whiteness stabilizer and titanium dioxide.
 11. The process according to claim 4 wherein a fibrous material treated with a whiteness stabilizer in the pulp, in a surface coating or by means of impregnation is coated with a brush-on color comprising titanium dioxide, or a non-treated fibrous material is coated with a brush-on color comprising a whiteness stabilizer and titanium dioxide.
 12. The process according to claim 6 wherein a fibrous material treated with a whiteness stabilizer in the pulp, in a surface coating or by means of impregnation is coated with a brush-on color comprising titanium dioxide, or a non-treated fibrous material is coated with a brush-on color comprising a whiteness stabilizer and titanium dioxide.
 13. The process according to claim 1 wherein said thiol (i) is an ester of a polyhydric alcohol selected from the group consisting of ethylene glycol, di-, tri- and polyethylene glycol, propane-1,2- and 1,3-diol, butane-1,4-diol, hexane-1,2,6-triol, trimethylolethane (TME), trimethylolpropane (TMP), erythritol, pentaerythritol (penta), di-TME, di-TMP, di-penta, sugar alcohols, sugar acids and lactones thereof and water-soluble polymeric alcohols, with a mercaptocarboxylic acid selected from the group consisting of mercaptoacetic acid (=thioglycollic acid), 2- and 3-mercaptopropionic acid, mono- and dimercaptosuccinic acid, wherein at least one hydroxyl group of the polyhydric alcohol is non-esterified.
 14. The process according to claim 2 wherein said thiol (i) is an ester of a polyhydric alcohol selected from the group consisting of ethylene glycol, di-, tri- and polyethylene glycol, propane-1,2- and 1,3-diol, butane-1,4-diol, hexane-1,2,6-triol, trimethylolethane (TME), trimethylolpropane (TMP), erythritol, pentaerythritol (penta), di-TME, di-TMP, di-penta, sugar alcohols, sugar acids and lactones thereof and water-soluble polymeric alcohols, with a mercaptocarboxylic acid selected from the group consisting of mercaptoacetic acid (=thioglycollic acid), 2- and 3-mercaptopropionic acid, mono- and dimercaptosuccinic acid, wherein at least one hydroxyl group of the polyhydric alcohol is non-esterified.
 15. The process according to claim 4 wherein said thiol (i) is an ester of a polyhydric alcohol selected from the group consisting of ethylene glycol, di-, tri- and polyethylene glycol, propane-1,2- and 1,3-diol, butane-1,4-diol, hexane-1,2,6-triol, trimethylolethane (TME), trimethylolpropane (TMP), erythritol, pentaerythritol (penta), di-TME, di-TMP, di-penta, sugar alcohols, sugar acids and lactones thereof and water-soluble polymeric alcohols, with a mercaptocarboxylic acid selected from the group consisting of mercaptoacetic acid (=thioglycollic acid), 2- and 3-mercaptopropionic acid, mono- and dimercaptosuccinic acid, wherein at least one hydroxyl group of the polyhydric alcohol is non-esterified.
 16. The process according to claim 6 wherein said thiol (i) is an ester of a polyhydric alcohol selected from the group consisting of ethylene glycol, di-, tri- and polyethylene glycol, propane-1,2- and 1,3-diol, butane-1,4-diol, hexane-1,2,6-triol, trimethylolethane (TME), trimethylolpropane (TMP), erythritol, pentaerythritol (penta), di-TME, di-TMP, di-penta, sugar alcohols, sugar acids and lactones thereof and water-soluble polymeric alcohols, with a mercaptocarboxylic acid selected from the group consisting of mercaptoacetic acid (=thioglycollic acid), 2- and 3-mercaptopropionic acid, mono- and dimercaptosuccinic acid, wherein at least one hydroxyl group of the polyhydric alcohol is non-esterified.
 17. The process according to claim 9 wherein said thiol (i) is an ester of a polyhydric alcohol selected from the group consisting of ethylene glycol, di-, tri- and polyethylene glycol, propane-1,2- and 1,3-diol, butane-1,4-diol, hexane-1,2,6-triol, trimethylolethane (TME), trimethylolpropane (TMP), erythritol, pentaerythritol (penta), di-TME, di-TMP, di-penta, sugar alcohols, sugar acids and lactones thereof and water-soluble polymeric alcohols, with a mercaptocarboxylic acid selected from the group consisting of mercaptoacetic acid (=thioglycollic acid), 2- and 3-mercaptopropionic acid, mono- and dimercaptosuccinic acid, wherein at least one hydroxyl group of the polyhydric alcohol is non-esterified.
 18. The process according to claim 6, wherein said ester is a di- or triester of pentaerythritol with mercaptoacetic acid or 1 or 2-mercaptopropionic acid, or a di-, tri- or tetraester of a pentitol or hexitol.
 19. The process according to claim 18 wherein the ester is selected from the group consisting of pentaerythritol trithiolglycollate said sorbitol tetrathioglycollate.
 20. The process according to claim 1 wherein said thiol compound is an ester of an alcohol with an allyl structural element, with a mercaptocarboxylic acid having 2 to 4 C atoms.
 21. The process according to claim 20 wherein said ester is cinnamyl alcohol thioglycollate.
 22. The process according to claim 1 wherein the compound is a mercapto-s-triazine, mercapto-triazole, mercapto-diazole, mercapto-1,3,4-thiadiazole compound, 2-mercapto-4,6-diamino-s-trazine, wherein the amino groups can be substituted by alkyl, cycloalkyl or alkyl.
 23. The process according to claim 1 wherein the thiol compound is a compound selected from the group consisting of 2-amino-4,6-bis polysulfido-s-triazine, bis-4,6-diamino-s-triazin-2-yl tetrasulfide, bis-5-amino-1,3,4-triadiazol-2-yl di- and tetrasulfide, wherein the amino groups can in each case be alkyl-, cycloalkyl- or allyl-substituted.
 24. The process according to claim 1 wherein the thiol an amino acid containing thiol groups, an N-acylated amino acid containing thiol groups, or an oligopeptide containing thiol groups.
 25. The process according to claim 24 wherein the compound is selected from the group consisting of cysterine, N-acetylcysterine and glutathione.
 26. Whiteness-stabilized lignin-containing fibrous material, of mechanically produced, thermomechanically, chemo-thermomechanically wood pulp or high-yield cellulose, containing a whiteness stabilizer selected from the group consisting of the thiols, di- and polysulfides according to groups (i) to (iv) of claim 1 in at least one of the pulp of the fibrous material and in a layer produced by impregnation, sizing or by a brush-on color.
 27. Whiteness-stabilized lignin-containing fibrous material according to claim 26, wherein the fibrous substance is a bleached wood pulp or high-yield cellulose.
 28. Whiteness-stabilized lignin-containing fibrous material according to claim 26 which has a coating of a size or coating color comprising titanium dioxide.
 29. Whiteness-stabilized lignin-containing fibrous material according to claim 27 which has a coating of a size or coating color comprising titanium dioxide.
 30. Whiteness-stabilized lignin-containing fibrous material according to claim 26 further comprising in the pulp and/or in a layer produced by impregnation and/or coating, one or more other whiteness stabilizers and/or co-stabilizers selected from the group consisting of agents which trap free radicals, UV absorbers fluorescent brighteners, antioxidants, metal deactivators, reducing agents, phosphites and other aromatic sulfides.
 31. A composition comprising a whiteness stabilizer, characterized by a content of at least one whiteness stabilizer according to claim 1, in an aqueous, organic or aqueous-organic liquid or paste-like medium which can be used for spraying or coating.
 32. A composition comprising a whiteness stabilizer, characterized by a content of at least one whiteness stabilizer according to claim 13, in an aqueous, organic or aqueous-organic liquid or paste-like medium which can be used for spraying or coating.
 33. Composition according to claim 31, further comprising one or more other whiteness stabilizers and/or co-stabilizers selected from the group consisting of agents which trap free radicals, antioxidants, UV absorbers, reducing agents, fluorescent brighteners, metal deactivators, phosphites and aromatic sulfides.
 34. Composition according to claim 31 further comprising a HALS light stabilizer, and/or an antioxidant from the series consisting of sterically hindered phenols.
 35. The composition according to claim 34 containing 2,2,6,6 tetramethyl-4-hydroxypiperidine N-oxide, and/or a 2-hydroxyaryl-benzotriazole a hydroxybenzophenone or a 2-substituted 3-(hydroxyaryl)acrylic acid ester.
 36. Composition according to claim 34 additionally comprising a HALS light stabilizer and a UV absorber in a weight ratio of 1 to (0.02 to 0.5) to (0.2 to 2), in particular 1 to (0.05 to 0.2) to (0.5 to 2).
 37. Composition according to claim 31 characterized in that it comprises a dispersing agent and/or an organic binder or sizing agent and/or titanium dioxide.
 38. Composition according to claim 31 characterized in that it is a brush-on color for coating a fibrous material.
 39. UV absorbers for use for stabilizing the whiteness of lignin-containing fibrous materials, characterized by a structural element of the formula

wherein: R¹: denotes H, (C₁-C₁₂)-alkanoyl, in particular acetyl, propionyl, n-butanoyl; (C₁-C₁₂) alkoxycarbonyl, in particular —COOCH₃, —COOC₂H₅, R²: denotes (C₁-C₁₂)-alkyl, linear or branched, in particular methyl, ethyl, n-propyl, n-butyl, i-propyl Ar—OH: denotes hydroxy-substituted aryl radical, wherein at least one hydroxyl group is bonded in the ortho- or para-position of the aromatic bonded to the olefinic double bond, in addition further substituents, such as alkyl, in particular methyl, ethyl, propyl, n-butyl, i-propyl, sec-butyl, tert-butyl, alkoxy, in particular methoxy and ethoxy, phenyl, p(o)-hydroxyphenyl, benzoyl, —COOH and SO₃H can be bonded to the aromatic, and wherein Ar represents the benzene or naphthalene ring.
 40. UV absorbers for use for stabilizing the whiteness of lignin-containing fibrous substances according to claim 39 characterized in that it is a 3-methoxy-4-hydroxybenzylidene-malonic acid dialkyl ester, wherein alkyl represents methyl or ethyl. 